Table cells are insets, but not from the point of view of DocIterators

and Cursors. So just calling InsetText::addToToc for the cells causes
problems, because InsetText::addToToc then adds the cell inset itself
as part of the DocIterator. This then leads to assertions, such as bug

The solution is to refactor InsetText::addToToc so that we can call the
iterating part without adding the inset.

src/insets/InsetTabular.cpp

@@ -3432,6 +3432,12 @@ docstring InsetTableCell::asString(bool intoInsets)
 }
 
 
+void InsetTableCell::addToToc(DocIterator const & di, bool output_active) const
+{
+	InsetText::iterateForToc(di, output_active);
+}
+
+
 docstring InsetTableCell::xhtml(XHTMLStream & xs, OutputParams const & rp) const
 {
 	if (!isFixedWidth)

src/insets/InsetTabular.h

@@ -70,6 +70,8 @@ public:
 	docstring asString(bool intoInsets = true);
 	///
 	docstring xhtml(XHTMLStream &, OutputParams const &) const;
+	///
+	void addToToc(DocIterator const & di, bool output_active) const;
 private:
 	/// unimplemented
 	InsetTableCell();

src/insets/InsetText.cpp

@@ -815,6 +815,13 @@ void InsetText::addToToc(DocIterator const & cdit, bool output_active) const
 {
 	DocIterator dit = cdit;
 	dit.push_back(CursorSlice(const_cast<InsetText &>(*this)));
+	iterateForToc(dit, output_active);
+}
+
+
+void InsetText::iterateForToc(DocIterator const & cdit, bool output_active) const
+{
+	DocIterator dit = cdit;
 	Toc & toc = buffer().tocBackend().toc("tableofcontents");
 
 	BufferParams const & bufparams = buffer_->params();

src/insets/InsetText.h

@@ -217,6 +217,8 @@ protected:
 	docstring getCaptionText(OutputParams const &) const;
 	///
 	docstring getCaptionHTML(OutputParams const &) const;
+	///
+	void iterateForToc(DocIterator const & cdit, bool output_active) const;
 private:
 	///
 	bool drawFrame_;